1. Field of the Invention
The present invention relates to a dynamic model parameter identification system for accurately estimating dynamic models required in dynamic control introduced in order to operate a robotic manipulator accurately and at high speed.
2. Background Art
As a result of developments in robotic technology, robotic manipulators have recently been used not only for spot welding and for inserting simple parts in assembly operations, but also for such tasks as arc welding, painting, and sealing-jobs that require high speed and high accuracy. Therefore, higher and more precise performance from robotic manipulators is desirable in order to increase the productivity of these operations. For this reason, it will be necessary to actively compensate the effect of nonlinear dynamics which can be significant in the high speed, high accuracy motion of robotic manipulators of the high reduction ratio type. However, it is not possible to sufficiently satisfy this demand with conventional methods wherein in, for example, independent proportional derivative control system is employed for each joint. With the objective of resolving this problem, there have been proposed a variety of dynamic control methods, the computed torque method being one representative of these. The principal task of these dynamic controls is to calculate the joint torques necessary for the control of given trajectories and at given speeds. Accordingly, when moving a robotic manipulator along a trajectory generated in real time using sensors or the like, it is necessary to carry out these calculations in real time at a high speed, and the sampling time required would be 15 times the eigen-frequency of the manipulator mechanism. With the recent developments of high-speed processors like DSP and of newly developed parallel processing algorithms, however, high-speed computation for dynamic control has become possible.
Accurate dynamic models for robotic manipulators are necessary for carrying out dynamic control. More concretely, in addition to such geometric parameters as link length, dynamic parameters such as link mass, inertial tensor, and first-order moment must be identified. Although geometric parameters can be identified according to static position and orientation measurements of the robotic manipulator, it is not possible to identify dynamic parameters by static measurements, and is still difficult even when the robotic manipulators are designed using such sophisticated design tools as CAD systems. Moreover, there are few commercial robotic manipulators whose dynamic parameters are publicly available.
In order to resolve such problems as mentioned above, research regarding the identification of dynamic parameters has been widely carried out. From this research there has been proposed a sequential identification method wherein parameters are sequentially identified from a plurality of tests which are determined according to the structural configuration of the robotic manipulator under consideration. Further, it has been clarified that it is possible to derive a linear input-output equation relating to dynamic parameters from equations of motion for robotic manipulators of the revolving joint type. In this method, dynamic parameters are identified by measuring joint torques and joint angular position, velocity and acceleration, when all the joints are driven at the same time. However, the fundamental problem with these methods is that there are parameters that do not contribute to the joint torques and that there are redundant parameters that only contribute as linear combinations with other parameters. For this reason the concept of base parameters as non redundant parameters was proposed, and research was carried out on the base parameters of serial link type robotic manipulators and base parameters of closed link type robotic manipulators. From this research, a procedure was proposed which used a covariance matrix of nonlinearity to derive the principal base parameters contributing predominantly to the joint torques.
With regard to the principal base parameters of serial link type and closed link type robotic manipulators, the history of theoretical study is still relatively new, and sufficient experimentation has not yet been carried out. Moreover, although there are some experimental studies that have been done using simple direct-drive manipulators with little friction, there are few studies of the highly geared manipulators widely used in industries. In these studies, the sequential identification method was applied to the basic three degrees of freedom of an industrial manipulator with DC motors and harmonic drive mechanisms implemented in its joints, and it was shown that the viscosity friction coefficient and Coulomb's friction coefficient are not reliant on the orientation of the robotic manipulator but are almost constant in the tested cases. Other experimental studies have been performed on the Puma 760. These concluded that the controlling parameters for torque were the Coulomb friction terms and the diagonal terms of the inertial matrix, and that, because of the large size of the manipulator, it was difficult to excite the inertial dependent parameters and, as a result, these parameters could not be separated from the gravitational parameters. The studies described so far have used the sequential identification method for which the test procedure is complicated but for which the identification operation is easily arranged. Moreover, when the sequential identification method is employed, it has also been pointed out that errors easily accumulate. In contrast, there are few corroborative reports for the simultaneous identification method, which is a superior method with respect to the fact that the necessary parameters can be simultaneously inferred. In particular, with regard to the simultaneous identification method, the preferable operation for accurate identification is not clear, and in a manipulator with multiple degrees of freedom, the number of dynamic model parameters inevitably increases and, thus, the identification becomes even more difficult. For this reason, in spite of the appearance of such high speed processors as DSP, the application of dynamic control to industrial robotic manipulators is still a difficult matter.
In the conventional art, because there is not means for intrinsically selecting the parameters which have a controlling effect on the control capacity, great labor cost is required to identify the parameters. Further, in the conventional art a means for easily providing the preferable operation for parameter identification is not provided, therefore the highly accurate identification of parameters is difficult.